The invention generally relates to controlling the temperature and relative humidity of an incoming fuel cell reactant stream.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, which permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:H2→2H++2e− at the anode of the cell, andO2+4H++4e−→2H2O at the cathode of the cell.
A typical fuel cell has a terminal voltage of up to about one volt DC. For purposes of producing much larger voltages, multiple fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow field plates (graphite composite or metal plates, as examples) that are stacked one on top of the other. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. A PEM is sandwiched between each anode and cathode flow field plate. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to act as a gas diffusion media and in some cases to provide a support for the fuel cell catalysts. In this manner, reactant gases from each side of the PEM may pass along the flow channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair of catalyst layers are often referred to as a membrane electrode assembly (MEA). An MEA sandwiched by adjacent GDL layers is often referred to as a membrane electrode unit (MEU).
A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. Thus, a controller of the fuel cell system may monitor the output power of the stack and based on the monitored output power, estimate the fuel flow to satisfy the appropriate stoichiometric ratios. In this manner, the controller regulates the fuel processor to produce this flow, and in response to controller detecting a change in the output power, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly.
The fuel cell system may provide power to a load, such as a load that is formed from residential appliances and electrical devices that may be selectively turned on and off to vary the power that is demanded by the load. Thus, the load may not be constant, but rather the power that is consumed by the load may vary over time and abruptly change in steps. For example, if the fuel cell system provides power to a house, different appliances/electrical devices of the house may be turned on and off at different times to cause the load to vary in a stepwise fashion over time.
Certain membranes used in PEM fuel cells, such as membranes derived from sulfonated fluorocarbon polymers, require the membrane to remain hydrated to function properly. In such systems, maintaining membrane hydration is critical to membrane longevity, reliability, performance, and even safety in some cases. In such systems, generally the less a membrane is hydrated, the less effective it will be at transporting ions from anode to cathode in the fuel cell reaction. Subsaturated membranes can also become brittle and can eventually develop holes through which the hydrogen and oxygen reactants can directly interact, which can exacerbate the problem through the heat generated from direct hydrogen oxidation, and in some cases, fire can even result in this manner if a membrane is allowed to operate without adequate hydration.
In such systems, the fuel cell operating temperature is generally about 50-100° C. (e.g., 80° C.). The fuel and oxidant reactant streams are generally supplied to the fuel cell at around this temperature range. Where reformate is used as a hydrogen fuel source, is generally saturated with water when it reaches the fuel cell since it has usually been cooled from a saturated state in a fuel processing reactor at a higher temperature. Where ambient air is used as an oxidant stream for a fuel cell, it is generally preheated and humidified with steam or by some other method such as an enthalpy wheel. In this context, a stream is referred to as saturated if it has a relative humidity of 100% for a given temperature. Similarly, the temperature at which a gas is saturated is referred to as the dew point temperature of the gas.
It will be appreciated that if a saturated reactant stream is cooled once it enters a fuel cell, then water will tend to condense from the stream. This can cause problems if the liquid water in the fuel cell blocks reactants from reaching the MEA, “flooding” the cell. Conversely, if a reactant stream is subsaturated as it enters a fuel cell, it will tend to remove water from the fuel cell as it passes through (e.g., either from the water produced at the cathode or from the membrane itself). Another factor affecting the water balance in a fuel cell is that the fuel cell reaction generally produces a variable amount of heat, depending for example on the operating condition of the membrane. For this reason, a coolant is generally circulated around a fuel cell to maintain the operating temperature of the fuel cell at a given point. Typically, the fuel cell temperature will increase from inlet to outlet by a degree affected by the flow rates, coolant temperature, etc.
Also, as previously indicated, one molecule of water is produced at the cathode for every molecule of hydrogen that is reacted at the anode. Thus, a significant amount of water is generated in a fuel cell as it is operated. Since hydrated fuel cell membranes such as Nafion are also generally very effective as water transport membranes, water can generally diffuse freely throughout a membrane as a fuel cell is operated. These factors and others can contribute to a very complex and dynamic water balance in and around a fuel cell.
There is a continuing need for fuel cell systems addressing concerns and objectives including the foregoing in a robust and cost effective manner.